src/devices/graphics/arch-gameloops.jd

page.title=Game Loops
@jd:body

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<p>A very popular way to implement a game loop looks like this:</p>

<pre>
while (playing) {
    advance state by one frame
    render the new frame
    sleep until it’s time to do the next frame
}
</pre>

<p>There are a few problems with this, the most fundamental being the idea that the
game can define what a "frame" is.  Different displays will refresh at different
rates, and that rate may vary over time.  If you generate frames faster than the
display can show them, you will have to drop one occasionally.  If you generate
them too slowly, SurfaceFlinger will periodically fail to find a new buffer to
acquire and will re-show the previous frame.  Both of these situations can
cause visible glitches.</p>

<p>What you need to do is match the display's frame rate, and advance game state
according to how much time has elapsed since the previous frame.  There are two
ways to go about this: (1) stuff the BufferQueue full and rely on the "swap
buffers" back-pressure; (2) use Choreographer (API 16+).</p>

<h2 id=stuffing>Queue stuffing</h2>

<p>This is very easy to implement: just swap buffers as fast as you can.  In early
versions of Android this could actually result in a penalty where
<code>SurfaceView#lockCanvas()</code> would put you to sleep for 100ms.  Now
it's paced by the BufferQueue, and the BufferQueue is emptied as quickly as
SurfaceFlinger is able.</p>

<p>One example of this approach can be seen in <a
href="https://code.google.com/p/android-breakout/">Android Breakout</a>.  It
uses GLSurfaceView, which runs in a loop that calls the application's
onDrawFrame() callback and then swaps the buffer.  If the BufferQueue is full,
the <code>eglSwapBuffers()</code> call will wait until a buffer is available.
Buffers become available when SurfaceFlinger releases them, which it does after
acquiring a new one for display.  Because this happens on VSYNC, your draw loop
timing will match the refresh rate.  Mostly.</p>

<p>There are a couple of problems with this approach.  First, the app is tied to
SurfaceFlinger activity, which is going to take different amounts of time
depending on how much work there is to do and whether it's fighting for CPU time
with other processes.  Since your game state advances according to the time
between buffer swaps, your animation won't update at a consistent rate.  When
running at 60fps with the inconsistencies averaged out over time, though, you
probably won't notice the bumps.</p>

<p>Second, the first couple of buffer swaps are going to happen very quickly
because the BufferQueue isn't full yet.  The computed time between frames will
be near zero, so the game will generate a few frames in which nothing happens.
In a game like Breakout, which updates the screen on every refresh, the queue is
always full except when a game is first starting (or un-paused), so the effect
isn't noticeable.  A game that pauses animation occasionally and then returns to
as-fast-as-possible mode might see odd hiccups.</p>

<h2 id=choreographer>Choreographer</h2>

<p>Choreographer allows you to set a callback that fires on the next VSYNC.  The
actual VSYNC time is passed in as an argument.  So even if your app doesn't wake
up right away, you still have an accurate picture of when the display refresh
period began.  Using this value, rather than the current time, yields a
consistent time source for your game state update logic.</p>

<p>Unfortunately, the fact that you get a callback after every VSYNC does not
guarantee that your callback will be executed in a timely fashion or that you
will be able to act upon it sufficiently swiftly.  Your app will need to detect
situations where it's falling behind and drop frames manually.</p>

<p>The "Record GL app" activity in Grafika provides an example of this.  On some
devices (e.g. Nexus 4 and Nexus 5), the activity will start dropping frames if
you just sit and watch.  The GL rendering is trivial, but occasionally the View
elements get redrawn, and the measure/layout pass can take a very long time if
the device has dropped into a reduced-power mode.  (According to systrace, it
takes 28ms instead of 6ms after the clocks slow on Android 4.4.  If you drag
your finger around the screen, it thinks you're interacting with the activity,
so the clock speeds stay high and you'll never drop a frame.)</p>

<p>The simple fix was to drop a frame in the Choreographer callback if the current
time is more than N milliseconds after the VSYNC time.  Ideally the value of N
is determined based on previously observed VSYNC intervals.  For example, if the
refresh period is 16.7ms (60fps), you might drop a frame if you're running more
than 15ms late.</p>

<p>If you watch "Record GL app" run, you will see the dropped-frame counter
increase, and even see a flash of red in the border when frames drop.  Unless
your eyes are very good, though, you won't see the animation stutter.  At 60fps,
the app can drop the occasional frame without anyone noticing so long as the
animation continues to advance at a constant rate.  How much you can get away
with depends to some extent on what you're drawing, the characteristics of the
display, and how good the person using the app is at detecting jank.</p>

<h2 id=thread>Thread management</h2>

<p>Generally speaking, if you're rendering onto a SurfaceView, GLSurfaceView, or
TextureView, you want to do that rendering in a dedicated thread.  Never do any
"heavy lifting" or anything that takes an indeterminate amount of time on the
UI thread.</p>

<p>Breakout and "Record GL app" use dedicated renderer threads, and they also
update animation state on that thread.  This is a reasonable approach so long as
game state can be updated quickly.</p>

<p>Other games separate the game logic and rendering completely.  If you had a
simple game that did nothing but move a block every 100ms, you could have a
dedicated thread that just did this:</p>

<pre>
    run() {
        Thread.sleep(100);
        synchronized (mLock) {
            moveBlock();
        }
    }
</pre>

<p>(You may want to base the sleep time off of a fixed clock to prevent drift --
sleep() isn't perfectly consistent, and moveBlock() takes a nonzero amount of
time -- but you get the idea.)</p>

<p>When the draw code wakes up, it just grabs the lock, gets the current position
of the block, releases the lock, and draws.  Instead of doing fractional
movement based on inter-frame delta times, you just have one thread that moves
things along and another thread that draws things wherever they happen to be
when the drawing starts.</p>

<p>For a scene with any complexity you'd want to create a list of upcoming events
sorted by wake time, and sleep until the next event is due, but it's the same
idea.</p>